1. Field of the Invention
The present invention relates to an orthogonal frequency division multiplexing (OFDM) receiver, and more particularly, to an OFDM receiver capable of implementing a frame structure having a small number of reference symbols and a large number of data symbols by selectively using time-domain and frequency-domain equalization schemes and improving OFDM signal reception performance and a time-domain equalizer capable of improving equalization performance.
2. Description of the Related Art
An orthogonal frequency division multiplexing (OFDM) scheme is a well-known high-speed data transmission scheme for the next-generation communication technology. In the OFDM transmission scheme, a sequence of symbols which are serially input is converted into parallel data (parallel symbols) in units of a predetermined block, and a plurality of the parallel symbols are multiplexed with different subcarrier frequencies. In a multi-carrier used for the OFDM scheme, there is a need for orthogonality between carriers. The orthogonality means that a product of two carriers is zero. The orthogonality is a necessary condition for the multi-carrier. The OFDM scheme is implemented by fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT). The OFDM scheme can be simply performed by using the orthogonality between the carriers and the definition of the FFT.
Particularly, in the OFDM scheme, each symbol has a cyclic prefix (CP). If delay spread caused from a channel is shorter than a length of the CP, all the subcarriers are maintained to have a frequency multiple of a reference frequency, so that only phase rotation caused from time delay occurs without inter-carrier interference (ICI). If a transmission signal used for an OFDM scheme is a differential encoding signal, a phase difference thereof can be detected at the time of demodulation, so that the influence of the RF channel can be minimized.
However, the maximum delay spread caused from an RF channel cannot always be shorter than the length of the CP. If the maximum delay spread becomes longer than the length of the CP, the ICI and inter-symbol interference (ISI) occur, so that reception performance of an OFDM receiver is greatly deteriorated.
In order to avoid the deterioration in the reception performance of the OFDM receiver, an OFDM equalization scheme may be used. The OFDM equalization scheme is mainly classified into a time-domain equalization scheme and a frequency-domain equalization scheme.
The frequency-domain equalization scheme has an advantage in that deformation of the subcarriers caused from a multi-path channel can be compensated. However, in use of the frequency-domain equalization scheme, a separate reference symbol (sometimes, referred to as a pilot symbol) is necessary used. In addition, as a distance of a data symbol from the reference symbol becomes longer (in a case where the data symbol is separated by 50 or more symbols from the reference symbol), equalization performance is deteriorated.
On the other hand, the time-domain equalization scheme has an advantage in that the equalization can be performed by using the CP included in the OFDM symbol without use of a separate reference symbol. However, if the maximum delay spread is large, effective equalization performance cannot be obtained. In addition, since only the errors in the shot time interval can be corrected, the equalization performance is lowered in comparison with the frequency-domain equalization scheme.
Particularly, the OFDM signal used for the currently-commercialized digital multimedia broadcasting (DMB) has a frame structure where one frame includes 76 symbols of which the first symbol is designated as the reference symbol. Therefore, if the frequency-domain equalization scheme is applied to the DMB, the equalization performance for a symbol separated by a long distance from the reference symbol is deteriorated. Although the time-domain equalization scheme is applied to the DMB, it is difficult to improve the equalization performance due to the shortcoming of the time-domain equalization.